The occurrence of dark (i.e. pigmented and/or stained) fibers often gives rise to annoying and expensive problems for manufacturers at all stages of fiber processing. For example, extensive literature is available on the occurrence of dark fibers in white wool, see e.g.: Fleet, M. R., Pigmented Fibres in White Wool, Wool Technology and Sheep Breeding 33, 5-13 (1985); Fleet, M. R., Stafford, J. E., Dawson, K. A., and Dolling, C. H. S., Contamination of White Wool by Melanin-pigmented Fibres when Pigmented and White Sheep Graze Together, Aust. J. Exp. Agric. 26, 159-163 (1986); Foulds, R. A., Wong, P., and Andrews, J. W., Dark Fibres and Their Economic Importance, Wool Technology and sheep Breeding 32(2), 91-100 (1984), and; Nolan, C., and Foulds, R., Dark-fibre Contamination in Wool, Queensland Agricultural J. Nov.-Dec., 305-307 (1985). The degree of contamination of white wool by colored fibers has a significant influence on its commercial value, especially when the wool is to be processed into light or pastel-colored articles. The manual removal of dark fibers is an extremely work- and cost- intensive, eye-straining job.
If the contents of dark fibers in white wool are above an acceptable level for white or pastel end uses, then those dark fibers need to be lightened to improve the appearance and to increase the value of the goods (see in this regard Turner, T. R., and Foulds, R. A., Decision Schemes for Assessing Dark Fiber Concentration in Top, Textiles Res. J. 57(12), 710-720 (1987). It is often found that the fibers and sliver of yarn are not tested properly for dark fiber content, and hence these impurities are first seen as dark fibers interwoven into the fabric matrix or in the end product. In such cases the dark fibers have to be removed manually with tweezers. A more convenient and economical alternative is given by the possibility of a wet treatment, which is much more productive and in many cases also less expensive.
The color of dark (i.e. pigmented) fibers ranges from black through shades of brown to light yellow, and the lightening of black fibers needs more severe wet treatment than those of the lighter fibers. Wet treatment conditions, however, should not be so severe as to damage the fibers excessively at the expense of lightening a few black fibers. Therefore, the present invention utilizes a treatment which is selective for areas of high dark fiber content. There have been numerous publications on the bleaching of hair (see e.g. Wolfram, L. J., and Albrecht, L., Chemical and Photo-bleaching of Brown and Red Hair, J. Soc. Cosmet, Chem. 82, 179-191 (1987); Wolfram, L. J., Hall, K., and Hui, I., The Mechanism of Hair Bleaching, J. Soc. Cosmet. Chem. 21, 875-900 (1970), and; Zahn, H., Hilterhaus, S., and Strussman, A., Bleaching and Permanent Waving Aspects of Hair Research, J. Soc. Cosmet. Chem. 37, 159-175 (1986)) and dark wool fibers (see for example, Bereck, A., Bleaching of Dark Fibres in Wool, Proc. 7th. Int. Wool Res. Conf., Tokyo, vol. IV, 152-162 (1985); Bereck, A., and Kaplin, J. J., Electron-microscope Observations on the Disintegration of Melanin Granules in Chemically Treated Karakul Wool, J. Textile Inst. 74, 44-47 (1983); Bereck, A., Zahn, H., and Schwarz, S., Das Selective Bleichen von Pigmentierten Haaren in Rohweisser Wolle, Textil Praxis Int. 37, 621-629 (1982) Finnimore, E., and Bereck, A., Verhalten von selectiv gebleichter Wolle, Melliand Textilberichte 68, 669-672 (English translation, E291-292) (1987); Kriel, W. J., Albertyn, D., and Swanepoel, O. A., Melanin-bleeding of Pigmented Karakul Wool, SAWTRI [South African Wool Textile Research Institute] Bulletin 3(1), 16-20 (1969); Laxer, G., and Whewell, C. S., Some Physical and Chemical Properties of Pigmented Animal Fibres, Proc. Int. Wool Res. Conf. Australia vol. F, 186-200 (1955); Teasdale, D. C., and Bereck, A., The Measurement of the Color of Bleached and Natural Karakul Wool, Textile Res. J. 51, 541-549 (1981), and; Van Heerden, N., Becker, J., van der Merwe, J. P., and Swanepoel, O. A., Bleaching of Karakul Wool, SAWTRI [South African Wool Textile Research Institute] Bulletin 3(4), 21-23 (1969)). Laxer and Whewell, Ibid, first realized that black-brown pigmented fibers absorb iron from ferrous sulfate solutions more rapidly and to a greater extent than white fibers, probably owing to the formation of a metal complex with the melanin of the pigment granules. Union between the iron and the fiber is reasonably firm and this bound iron is a useful catalyst for promoting bleaching when the iron-containing fibers are immersed in solutions of hydrogen peroxide.
All known processes for bleaching pigmented dark fibers are based on the use of peroxy compounds, Bereck (1985), Ibid. Wolfram et al (1970), Ibid, have studied the mechanism of hair bleaching in detail. They found that the bleaching reaction occurs in two steps; the initial solubilization of the granules is followed by the decolorization of the dark brown solubilized pigment. The pigment granules are distributed within the cortex (Laxer, Ibid) and therefore the bleaching of the granules is a diffusion-controlled reaction. Some oxidation of the keratin matrix does occur during the bleaching process due to diffusion. Wolfram et at (1970) Ibid, showed that neither reducing agents such as thioglycolic acid; borohydride, sulfide and sulfite, nor some oxidizing agents such as persulfate, perchlorate, iodate and permanganate, produce any apparent physical change in the melanin pigment. A different behavior was displayed by hydrogen peroxide. Dilute aqueous solutions of this reagent caused disintegration of the pigment granules, which slowly dissolved in the reaction system. The dark brown solution gradually became lighter over a long period of time. The second step (decolorization of the melanin granules) is therefore much slower than the first step (solubilization of the melanin pigment) and hence the former is the rate-determining step in the overall process. It was pointed out that the disintegration process alone is unlikely to affect the color of hair significantly; it may cause only a slight change in hue.
The dissolution of melanin in alkali, observed for example in the "bleeding" of pigmented fibers even at only sightly alkaline pH, is a well-known phenomenon, Kriel et al, Ibid. Bereck and Kaplin, Ibid, have studied the disintegration of melanin granules in chemically treated karakul wool using an electron microscope. Their studies revealed the following interesting features. Under identical bleaching conditions, the destruction of the melanin granules was virtually complete in the mordanted wool whereas in the untreated wool the granules were only partly dissolved. These workers have also observed that the electron micrographs of bleached wool were not unlike those of the samples treated with alkali. However, the change in luminosity due to the alkali treatment was negligible compared with the relatively high luminosity of the bleached wool. This strongly supports the view of Wolfram et al. (1970), Ibid, that melanin disintegration does not significantly influence fiber color. It may be said that the solubilized melanin stains the fibers in the same way as a black dyestuff, Bereck and Kaplin, Ibid. A mixture of hydrogen peroxide and ammonium and/or potassium persulfate has been used successfully in the bleaching of melanin granules, as described in Corbett, J. F., The Chemistry of Hair-care Products, J. Soc. Dyers Colour. 92, 285-303 (1976).
There had been extensive research carried out on the selective bleaching of dark fibers using Bereck's iron mordanting technique (as described in Bereck (1985), Ibid), and the process was adopted successfully by many West German textile mills. This process consists of 3 stages, namely (i) mordanting, (ii) rinsing, and (iii) bleaching. Bereck particularly pointed out the importance of a proper choice of reducing agents in the application of ferrous salts to wool during mordanting and the thorough rinsing of the "loosely bound" ferrous and ferric ions from wool. Of the many reducing agents tested in Bereck (1985), hypophosphorous and phosphorous acids proved to be the best stabilizing agents for minimizing damage to the wool fiber. Giesen and Ziegler in Die Absorption von Eisen durch Wolle und Haar, Melliand Textilberichte, 62, 482-283 (English translation, E622-625) (1981), provide a study of the absorption of iron by wool and hair and concluded that optimum conditions for selective absorption of iron by dark fibers in wool were achieved within a pH range of 3.0-3.5, using a treatment time of 60 minutes at 80.degree. C. Within the pH range mentioned above, the pigmented karakul wool absorbed the greatest amount of iron. At higher pH values, the absorption of iron by pigmented karakul wool diminished as the maximum uptake of iron by nonpigmented merino wool was reached at pH 4.5. Here, it would be disadvantageous to work at pH values greater than 3.5 due to an increase in iron uptake by nonpigmented wool, which may cause extensive damage and discoloration during bleaching.
Even though the aforementioned three-step process may be carefully conducted, there always remains some residual trivalent iron, which tends to give an overall undesirable reddish-brown discoloration or cast to the wool (apparently due to oxidation of ferrous to ferric ions during bleaching). Bereck et al 1982, Ibid, already have shown that selective bleaching hardly alters the natural cream color of wool. However, increasing demand for "bleached white" material led Finnimore and Bereck, Ibid, to investigate the further bleaching of selectively bleached material. Selectively bleached wool was given a second step reductive or oxidative bleaching to yield whiter material.
German Offenlegungsschrift 3,433,926 (3/27/86) to Streit et al discloses a single bath reductive and oxidative bleaching process, in which the reductive bleaching with thiourea dioxide precedes an oxidative hydrogen peroxide bleaching, whereas in the processes of the present invention the reductive bleaching is subsequent to the oxidative bleaching. Japanese patent 51-64082 (6/3/76) is drawn to a reductive bleaching process in which hydrogen peroxide and thiourea are mixed at the start of the bleaching processes (i.e., bleaching with a single mixture which contains both hydrogen peroxide and thiourea), while by contrast the instant invention utilizes separate steps of oxidative bleaching followed by reductive bleaching. It has unexpectedly and surprisingly been discovered that the process of the present invention provides greatly improved results (including, a higher Whiteness Index, lower Yellowness Index, and lower degree of damage) as compared to the results achieved by either of these two prior art processes.
It is a first object of the present invention to provide bleaching greatly superior to that of prior art processes, said bleaching providing fibers which are essentially pigment free, essentially free of iron residue (i.e. without the aforementioned undesirable reddish-brown discoloration or cast) and/or of a surprising and unexpectedly high degree of whiteness, low degree of yellowness and low degree of fiber damage.
It is a second object of the present invention to provide processes which may provide oxidative and reductive bleaching in a single bath, and thereby provide the advantages of: (a) avoiding the two or three step treatment processes normally required by conventional processes, thereby simplifying the process; (b) reducing the amount of time and energy required to provide effective bleaching; and (c) reducing the amount of equipment required to perform the bleaching.
Other objects and advantages of this invention will become readily apparent from the ensuing description.
The aforementioned objects and advantages are achieved by several processes of the instant invention. Two processes of the instant invention which employ mordanting utilize the initial steps of:
bringing both pigmented and unpigmented fibers into contact with ferrous ions under conditions which provide adsorption of the ferrous ions by the pigmented and unpigmented fibers; removing (as for example by rinsing) a portion of the ferrous ions from the pigmented and unpigmented fibers with at least a portion of the ferrous ions remaining on the pigmented fibers, and;
contacting the pigmented and unpigmented fibers with hydrogen peroxide under conditions which provide oxidative bleaching of both the pigmented and unpigmented fibers, including oxidative bleaching of the pigmented fibers by interaction of the hydrogen peroxide with ferrous ions remaining on the pigmented fibers, to produce bleached fibers in contact with unspent hydrogen peroxide. In a first process of the present invention said initial steps are followed by the steps of:
adding to the bleached fibers in contact with unspent hydrogen peroxide a material which combines with hydrogen peroxide to form a reductive bleaching agent in an amount sufficient to produce a reductive bleaching media; and
maintaining the bleached fibers in the reductive bleaching media under conditions providing reductive bleaching of the bleached fibers. In a second process of the present invention said initial steps are followed by the steps of:
adding to the bleached fibers in contact with unspent hydrogen peroxide, an inactivating material in an amount at least sufficient to inactivate all of said unspent hydrogen peroxide to form an inactivated media; and
subsequent to said inactivation of all said unspent hydrogen peroxide, reductively bleaching said bleached fibers by addition of a reductive bleaching agent to said inactivated media.
Additionally the present invention encompasses processes employing hydrogen peroxide and at least one persulfate containing compound, rather than the aforementioned iron-mordanting i.e.: first process which comprises,
contacting fibers with hydrogen peroxide and at least one persulfate containing compound under conditions which provide oxidative bleaching of the fibers to produce bleached fibers in contact with unspent hydrogen peroxide;
adding to the bleached fibers in contact with the unspent hydrogen peroxide (from the previous step, a material which combines with hydrogen peroxide to form reductive bleaching agent (e.g. thiourea, substituted thiourea (e.g. 1,3-dimethyl-2-thiourea, 1,3-diphenyl-2-thiourea, 1,1,3,3,-tetramethyl-2-thiourea), compounds containing thiol (for example, 1-dodecanethiol, 1-octadecanethiol, thioglycolic acid, thiophenol)), in an amount sufficient to produce a reductive bleaching media; and
maintaining the oxidatively bleached fibers in said reductive bleaching media under conditions providing reductive bleaching of the bleached fibers, and;
A second process of the present invention which comprises,
contacting fibers with hydrogen peroxide and at least one persulfate containing compound under conditions which provide oxidative bleaching of the fibers to produce bleached fibers in contact with unspent hydrogen peroxide;
adding to the bleached fibers in contact with unspent hydrogen peroxide (from the previous step-, an inactivating material in an amount at least sufficient to inactivate all of the unspent hydrogen peroxide to form an inactivated media; and
subsequent to the inactivation of all the unspent hydrogen peroxide, reductively bleaching the bleached fibers by addition of a reductive bleaching agent to the inactivated media.
The aforementioned processes unexpectedly and surprisingly provide fibers of superior whiteness, and by virtue of preventing deposition of ferric species provide fibers having surprising, highly advantageous and desirable properties e.g. fibers which are essentially pigment free as well as stain-free, essentially free of iron residue (thereby avoiding the aforementioned undesirable reddish-brown cast) and characterized by a high degree of whiteness with low degree of damage.